FYI: Flash Flood Guidance (FFG) provides a general indication
of the amount of rainfall evenly distributed over various durations (in
hours), necessary to cause small streams to overflow their natural banks.
During heavy rainfall, accurate interpretation of the guidance consists
of the combination of rainfall that has already occurred plus any additional
rainfall that can be confidently expected to fall over the same location
in the immediate future.

It should be noted that FFG values represent an estimate
of average soil moisture and stream flow conditions in a given county.
Flooding may occur with less rainfall than indicated in areas where recent
rain has been locally heavier. In addition, during prolonged dry spells,
soils may temporarily have impervious characteristics at the onset of
heavy rain, resulting in extra runoff and possible flooding from lower
rainfall amounts than those indicated by the FFG.

Headwater Flood Guidance

Headwater Flood Guidance depicts the approximate rainfall
evenly distributed over various durations to cause selected streams to
reach official NWS flood stage (may not necessarily coincide with the
natural bank) at U. S. Geological Survey gauging stations. As with Flash
Flood Guidance, these values are intended as general guidance.